Method of making a device for monitoring fatigue life

ABSTRACT

A device for monitoring the fatigue life of a structural member is comprised of at least one and preferably a plurality of substantially flat, elongated coupons which are fabricated of the same material as that of the member being monitored. The coupons are secured in parallel to the member so that they all experience the same strain history as the member. Each of the coupons includes a different stress concentrating notch pattern so that the application of the same strain to all of the coupons results in the development of different stress concentrations within the coupons. The development of different stress concentrations within the coupons causes each coupon to have a different fatigue life, the fatigue life of each coupon being a predetermined percentage of the fatigue life of the structural member being monitored.

This is a division of application Ser. No. 573,081 filed Jan. 23, 1984,now U.S. Pat. No. 4,590,804.

BACKGROUND OF THE INVENTION

The present invention relates to fatigue monitoring and, moreparticularly, to a device for providing an indication of the progressionof fatigue damage within a structure and a method for making such adevice.

Potential structural failure due to fatigue constitutes one of the mosttroublesome areas of structural engineering primarily because fatiguefailure occurs suddenly, usually in critical areas of a structure.Although many aspects of fatigue are still unknown, it is generallyunderstood that the fatigue process starts with the microscopicimperfections or defects which are present in all materials. Undercertain circumstances, the microscopic imperfections rapidly grow andcoalesce to form a macroscopic defect in the form of a crack. The growthand propagation of macroscopic cracks is the immediate cause of afatigue failure. However, the appearance of such macroscopic cracksoccurs relatively late in the fatigue process and, therefore, cannot beused as an acceptable warning device of impending fatigue failure.

The primary factor which causes the inherent microscopic materialdefects to grow and coalesce is the presence of an intense stress orstrain field (hereinafter collectively referred to as a "stress field").Such intense stress fields or stress concentrations generally occur inthe vicinity of sudden discontinuities or "stress raisers" such asholes, notches or other similar geometric discontinuities within astructural configuration. Thus, fatigue failure generally originates ator near such stress raisers and is believed to begin whenever a certaincritical stress or critical strain is exceeded. Fatigue is therefore aprimary design consideration in many applications which involve repeatedand often varying loadings such as aircraft, machine elements, pressurevessels, bridges, etc.

In the past, structural designers have attempted to circumvent theproblem of fatigue failure by designing structures in a manner whichmaintains the stresses present in the critical areas of a structure at alevel well below the known endurance limits of the material employed.Hence, minimum radius holes, fillets etc. are introduced into structuraldesigns and only "mild" stress raisers are employed in the structure inorder to provide relatively smooth structures and to thereby decreasethe likelihood of fatigue failure. While this type of design approachresults in structures which are generally safe and relatively free offatigue failure, it also results in unacceptable penalties in structuralefficiency which, in turn, result in excessive structural costs.

When designing a structure to compensate for fatigue and in trying toassess the potential fatigue life of the structure, a designer generallyrelies on a combination of experimental data and previously developedempirical design rules. Experimental fatigue data have been accumulatedon most structural materials in the form of S-n curves which provide anindication of the number of loading cycles which will induce fatiguefailure as a function of the stress level applied to the material. Theexperimental data, however, exhibit a significant amount of scatter andare only strictly applicable to structures where the cyclic stressapplied to the structure is of a constant amplitude. Most actualstructures which are subjected to repeated loadings generally experiencevarying levels of stress for different numbers of cycles. Thus, thefatigue life of a particular structure greatly depends upon its specificindividual stress history.

In applying the experimental S-n curve data to actual loadingsituations, empirical rules and procedures have been suggested. Theseempirical rules, referred to as cumulative damage rules, (the mostcommon of which is Minor's Rule), have also been found to be inadequate.Even in the simplest case of two different stress levels applied to astructure, it has been demonstrated experimentally that structuralfatigue life is dependent on the order of application of the stresslevels. None of the known cumulative damage rules take into account thepotential differences in the order of application to the structure ofdiffering stress levels. Moreover, cumulative fatigue damage cannot bedetermined by non-destructive testing so that, short of conducting adetailed microscopic examination of the structure of the material, thereis no known way to accurately determine the cumulative damage of astructural member at a given time.

In summary, in addition to being inefficient, the practice of designingstructures to take into account fatigue is highly speculative since theactual loading history of the structure is not known and cannot beaccurately predicted. Therefore, there is a need for a device whichwould monitor fatigue damage and provide a reliable estimate of theremaining fatigue life of a particular structure in order to provide awarning of impending fatigue failure in sufficient time to permitmeasures to be taken, such as the repair or replacement of suchstructures, to minimize the possibility of catastrophic consequences.

The present invention comprises a relatively simple fatigue monitoringdevice which can provide a repeatable, reliable estimate of theremaining fatigue life for any desired structural member. The device isattached to the structural member whose fatigue life is to be monitoredso that the device is subjected to the exact same strain history andenvironment experienced by the actual structural member. The device issmall and inexpensive to manufacture and can be specifically tailored toparticular structural applications and structural materials as required.

SUMMARY OF THE INVENTION

Briefly stated, the present invention is a device for monitoring thefatigue life of a member. This device is comprised of at least one andpreferable a plurality of substantially flat, elongated couponsfabricated of the same material as the member being monitored, thecoupons being mounted in parallel on the member so that all of thecoupons experience the same strain history as the member beingmonitored. Each of the coupons includes a special notch patterncomprised of at least one pair of notches designed to produce a localstress concentration. One notch of each of the notch pairs is disposedon each of the longitudinal sides of the coupon, the notches of thenotch pair being substantially geometrically the same. Their axis mustbe oriented along a suitably chosen direction. The notch pattern of eachof the coupons produces a stress field which varies in intensity fromrelatively mild to very severe. The severity of the local stress fieldis controlled by the geometry of the notch pattern. Smooth geometriesproduce mild stress concentration, while sharp geometric discontinuitiesproduce severe stresses. In this manner, the application of the samestrain history to all of the coupons results in the development ofdifferent stress concentrations in the region of the notch tips of eachcoupon, so that each coupon has a different fatigue life. The fatiguelife of each coupon is a percentage of the fatigue life of the memberbeing monitored. The present invention also comprises a method formaking the previously described device.

BRIEF DESCRIPTION OF THE DRAWING

The foregoing summary, as well as the following detailed descriptionwill be better understood when read in conjunction with the appendeddrawing. For the purpose of illustrating the invention, there is shownin the drawing an embodiment which is presently preferred, it beingunderstood, however, that this invention is not limited to the precisearrangement shown. In the drawing:

FIG. 1 is a plan view of five generally parallel coupons which comprisea preferred embodiment of a fatigue monitoring device in accordance withthe present invention;

FIG. 2 is an enlarged plan view representation of one of the coupons ofFIG. 1; and

FIG. 3 is a graphic representation of a portion of a typical load cycleversus stress amplitude relationship (S-n curve) for two of the couponsof FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to FIG. 1, there is shown a device 10 for monitoring thefatigue life of a structural member (not shown). The fatigue monitoringdevice 10 is comprised of a plurality of individual coupons 12, 14, 16,18 and 20, five such coupons being presently preferred. The coupons 12,14, 16, 18 and 20 are substantially flat (preferably about 0.5 mm inthickness) and are generally rectangular in shape with a longitudinallength of about 25 mm and a lateral width of about 6.5 mm. While thepreceding dimensions for the coupons are presently preferred, it shouldbe understood that the length, width and thickness of the coupons mayvary for a particular application, the present invention not beinglimited to coupons of any particular dimensions.

The coupons 12, 14, 16, 18 and 20 are preferably fabricated of the samematerial as the structural member (not shown) which is being monitored.In the presently preferred embodiment, the structural member beingmonitored (not shown) is comprised of a high strength aluminum alloyknown as 7075-T6 aluminum which is used extensively in the constructionof aircraft and other structures which require high strength and lightweight. The use of 7075-T6 aluminum in connection with the presentlypreferred embodiment is only for purposes of illustrating the principlesof the present invention it being clearly understood that the couponsmay be fabricated of any other material, such as other metals, plasticetc., preferably the same material as that employed in the particularstructural member to be monitored.

The coupons 12, 14, 16, 18 and 20 are combined into a single gauge whichis located in such a manner as to experience the same actual strainhistory and environment as is experienced by the structural member (notshown) which is being monitored. Generally, this means that the couponsare mounted on or secured directly to the structural element beingmonitored and remain with the structural element throughout its servicelife (not shown). The coupons may be secured to the member by pins orany other suitable type of bond such as an adhesive bond or spot weld.In order to assure that each of the coupons experiences the same strainhistory, the coupons should be mounted or attached to each other inparallel and secured to the member being monitored. The device need notbe mounted at the critical section of the structural element beingmonitored but should be mounted in such a manner as to experience thesame strain history and environment as the member. Preferably, thecoupon axes are oriented in the direction of the maximum principaltensile strain which is expected to be experienced by the member beingmonitored. By locating the coupons as described, the previouslydescribed difficulties involved in determining the cumulative damage ofthe structural member being monitored are avoided.

The basic premise of the invention is to insure that, since the basicprinciples of fatigue are not clearly understood in sufficient detail toaccurately determine cumulative damage and to predict remaining fatiguelife, fatigue should be monitored in such a manner that the monitoringdevice 10 has the same characteristics as the member being monitored andexperiences the same strain history and environment. However,accelerated fatigue damage is produced within the device 10 byintroducing into each coupon 12, 14, 16, 18 and 20 a predeterminedstress raiser or stress concentrating notch pattern which produces anintense stress concentration or stress intensity field within thecoupon. The introduction of such a stress raiser insures that each ofthe coupons has a shorter fatigue life than the structural member beingmonitored, assuming that the coupons and the member are subjected to thesame strain history. By varying the severity of the stress raisers (i.e.varying the notch pattern) within the individual coupons, each couponcan be specifically tailored to experience fatigue failure at a fairlyaccurate and repeatable portion or percentage of the expected life ofthe structural member being monitored. Thus, the failure of each couponprovides a generally reliable indication of the expiration of a portionof the fatigue life of the structural member being monitored.

The fatigue failure of the individual coupons can be detected in anysuitable manner, for example, by visual inspection or by the use of aremote means, to provide an indication of the portion or percentage ofthe fatigue life of the structural member being monitored which hasexpired and to thereby indicate the expected remaining fatigue life ofthe structural member or to give warning of impending fatigue failure.If the severity of the stress raisers introduced into the coupons isvaried over a suitable range extending from slightly more severe thanthe stress raisers present in the member being monitored and increasingin severity in steps, the fatigue life of the member can be monitoredrelatively accurately. As will hereinafter become apparent, the severityof the stress raisers is controlled primarily by the geometry of thenotches within the coupons. As will also hereinafter become apparent thegeometry of the stress raisers varies depending upon the particularmaterial of the structural member being monitored and may bepredetermined in accordance with the principles of this invention ashereinafter set forth in detail.

The coupons 12, 14, 16, 18 and 20 of the present embodiment are arrangedin order with the severity of the notch pattern increasing from left toright (i.e. the severity of the notch pattern in coupon 14 is greaterthan that of coupon 12 etc.). Thus, the coupons 12, 14, 16, 18 and 20 ofthe present embodiment are expected to reach their respective fatiguefailure points in the reverse order (i.e. coupon 20 will fail firstetc.). The severity of the notch pattern ranges from relatively mild,having no significant geometric discontinuities as shown in the smooth,semicircular notches 22 of coupon 12, to relatively severe, having aplurality of sharp geometric discontinuities as indicated by themultiple V-shaped notches of coupon 20.

The primary reason for the variations in the notch pattern is thesignificant difference in physical behavior and mathematical descriptionof the stresses which are experienced by the coupon in the vicinity ofthe notches. In the case of a coupon 12 having mild notches, the areanear the notch tips experience a local increase in stress varying fromthe undisturbed stress to a maximum occurring at the edge of the notch.This maximum value can be expressed by the equation:

    σ.sub.y =cσ

where:

σ_(y) is the stress present at the notch tip which is the maximum stressin the coupons;

σ is the uniform tension applied upon the ends of the coupon; and

C is a stress concentration factor.

The variation of the stress from its maximum value to the undisturbedvalue σ is a function which depends on the exact notch geometry. Forsemi-circular or semi-elliptical notches that function is a polynomial.

In contrast to the coupon 12 having a mild notch pattern, the stressesintroduced into a coupon 20 having a severe notch pattern consisting ofsharp discontinuities in the boundaries is given by an equation of thistype:

    σ.sub.n =K/X.sup.n

where:

σ_(n) is the stress along the line between the notches;

X is a coordinate with origin at the notch tip and directed along theline between notch tips;

K is a stress intensity factor; and

n denotes the rate at which the stresses increase near the notch tip(this is often called the order of the stress singularity).

As is apparent from the foregoing equation, in a coupon having a severenotch pattern, the stress tends to become infinite at the notch tip andwould do so if there were no limit to the strength of the material. Inreality this stress is limited by the material causing either the notchto propagate as a crack or the creation of a plastic zone in whichfatigue damage accumulates rapidly.

It has been found experimentally that the features of the stress nearbetween the notches are controlled by either the stress concentrationfactor (for mild notches) or the stress intensity factor (for severenotches) and by the rate at which the stresses decay back to theiraverage (undisturbed) value as the distance from the notches isincreased. These two factors control the rate at which damageaccumulates in a coupon near a notch and, therefore, the time requiredto produce a macro crack and a subsequent fatigue failure within thecoupon. Thus, given that a certain critical stress is required to causea microscopic defect within the coupon to grow, the stress intensity orstress concentration factor governs the overall magnitude of the stressfield created while the rate of decay of the stresses determines howclose to the notch a defect must be in order to cause the defect togrow. Assuming that the microscopic defects are distributed in astatistically homogeneous fashion throughout the coupon material, thecombination of the two factors can be viewed as a measure of theprobability that fatigue damage will occur within the coupon in the areabetween the notches. The desired result is to obtain a sequence ofpractical notch patterns within the coupons which provide control ofthese critical factors to insure that the fatigue lives of the couponsare all different but all significantly shorter than the fatigue life ofthe structural member being monitored.

It has been found that certain geometric features of the notches can bechanged to vary the severity of the notch pattern. For example, it hasbeen found that if the notches are aligned with each other at a zerodegree orientation angle θ as shown in coupon 16 the critical parametersare relatively insensitive to changes in the wedge angle α of thenotches. On the other hand, if the notches are positioned to be alignedat an orientation angle of 45 degrees from the perpendicular as shown oncoupon 18, the loading between the notches is in shear rather than intension and the rate of decay of the stresses is strongly affected bythe wedge angle of the two notches. Thus, the two controlling parameterswith respect to the variations in the notch pattern are the notch orwedge angle α and the orientation of the notches or orientation angle θ.By using a series of notches with differing wedge angles and orientationangles, the fatigue life of the individual coupons can be varied andcontrolled to provide coupons with fatigue lives which are differentpredetermined percentages of the fatigue life of the structural memberbeing monitored.

Viewing FIG. 1 it can be seen that the coupons 12, 14, 16, 18 and 20each have a different notch pattern. Coupon 12 has the mildest notchpattern which comprises a single pair of notches 22 and 24 which areeach substantially semicircular in shape and of the same size so thatthe notches 22 and 24 are substantially geometrically the same. Inaddition, the notches 22 and 24 are substantially aligned with eachother along a single laterally extending axis 26 to provide a zerodegree orientation angle. One of the notches is disposed on each of thelongitudinal sides of the coupon 12 as shown. As previously discussed,semicircular notches 22 and 24 or this type are considered to be mildstress raisers in that they are relatively smooth and contain no sharpgeometric discontinuities. Thus, in accordance with the foregoingdiscussion, coupon 12 can be expected to have a fatigue liferepresentative of a structural element with a mild stress raiser andthis life will be longer than the fatigue life of any of the othercoupons which will hereinafter be described.

Coupon 14 similarly includes a relatively mild notch pattern comprisedof single pair of notches 28 and 30, one of which is disposed on each ofthe longitudinal sides of the coupon 14. Again, the notches 28 and 30are directly aligned along a single lateral axis 32 to provide a zerodegree orientation angle. However, unlike the semicircular notches 22and 24 of coupon 12, the two notches of coupon 14 are each substantiallysemi-elliptically shaped both being of the same approximate size. Whilestill considered to be a mild stress raiser, the semi-ellipticallyshaped notches 28 and 30 have a higher stress calculation factor thanthe smooth semicircular shaped notches 22 and 24 of coupon 12.Therefore, when subjected to the same stress history, coupon 14 can beexpected to have a shorter fatigue life than that of coupon 12. Ofcourse, the fatigue life of coupon 14 also constitutes an approximatepredetermined percentage of the fatigue life of the structural memberbeing monitored.

Coupon 16 also includes a notch pattern comprised of a single pair ofnotches 34 and 36 disposed on each of the longitudinal sides of thecoupon. As with the notches of coupons 12 and 14, the notches 34 and 36are generally aligned along a single axis 38 (zero degree orientationangle) which is perpendicular to the longitudinal coupon sides as shown.However, unlike the notches of coupons 12 and 14, notches 34 and 36 arewedge or V-shaped to provide a relatively sharp geometric discontinuity.In the present embodiment the wedge angle α of notches 34 and 36 is 60degrees, it being understood that the present invention is not limitedto notches having a particular wedge angle. The stress fieldconcentration between the notches 34 and 36 is greater than that ofcoupons 12 and 14, so coupon 16 can be expected to have a shorterfatigue life than that of either coupon 12 or 14.

Coupon 18, which is shown in greater detail in FIG. 2, has a notchpattern comprised of a single pair of notches 40 and 42 each of which isgenerally V-shaped with a 60 degree wedge angle. Unlike the previouslydiscussed coupons, the notches of coupon 18 are aligned with each otherat approximately a 45 degree angle with respect to the lateral axis 44extending across the coupon 18. It can be expected that coupon 18 willhave a shorter fatigue life than any of the coupons 12, 14 or 16previously described.

Coupon 20 has a notch pattern comprised of two pairs of notches 46/48and 50/52. Each of the notches is generally V-shaped with a 60 degreewedge angle. The notches of each pair are respectively aligned atorientation angles of plus or minus 45 degrees from the lateral axis ofthe coupon 54. The notch pattern of coupon 20 produces the greateststress field intensity in the area between the notch pairs and,therefore, coupon 20 can be expected to have a shorter fatigue life thanany of the other coupons 12, 14, 16 or 18.

Coupons 12, 14, 16, 18 and 20 fabricated of 7075 aluminum wereconstructed and employed to monitor the fatigue life of a structuralmember which was also constructed of 7075 aluminum. The coupons 12, 14,16, 18 and 20 were combined into a gauge and mounted on a structuralmember which was also constructed of 7075 aluminum. When a fatigue testof the member was conducted of the beam with the device mounted on it,failure of the various coupons occurred at the number of cyclesindicated in the following table:

    ______________________________________                                        Coupon #   Cycles to Coupon Fatigue Failure                                   ______________________________________                                        12         371,600 cycles                                                     14         286,700 cycles                                                     16         203,900 cycles                                                     18         174,600 cycles                                                     20         163,600 cycles                                                     ______________________________________                                    

The structural member being monitored suffered fatigue failure at486,000 cycles. Examination of the test data indicates that couponsprovided fatigue damage warnings at approximately 33% (coupons 18 and20), 60% (coupons 14 and 16) and 75% (coupon 12) of the actual fatiguelife of the structural member being monitored. Additional testingverified that the results set forth above are repeatable to provide arelatively reliable indication of remaining fatigue life. A portion of atypical S-n curve for coupons 20 and 14 are shown in FIG. 3, line 56representing the S-n curve for coupon 20 and line 58 representing theS-n curve for coupon 14. As previously discussed, S-n curves provide anindication of the number of load cycles which will induce fatiguefailure as a function of the applied stress level.

Although the foregoing description of a preferred embodiment of thepresent invention relates to a set of coupons 12, 14, 16,18 and 20fabricated of 7075 aluminum for the purpose of monitoring the fatiguelife of a structural member of 7075 aluminum, it will be appreciatedthat the same concepts and techniques are applicable with respect toother materials. It will also be appreciated that the notch patterns,including the number of notch pairs, the geometry of the notches, thewedge angles α and the notch orientation angles θ may vary with othermaterials. Coupons may be produced utilizing the below described methodfor use in connection with other materials which are subject to fatiguefailure.

In addition, although the preferred embodiment employs a set of fivecoupons it should be appreciated that a greater or lesser number ofcoupons could be utilized, depending upon the particular application.For example, in a particular situation it may be necessary to identifyonly the 60 percent point of the fatigue life of a particular member. Insuch an application only a single coupon having a notch pattern whichindicates the expiration of approximately 60% of the life of the membercould be employed. However, it is preferable to employ more than asingle coupon in order to show progressive damage, to guard against anyundetected severe defect which might be present in the structural memberand generally to provide added confidence and safety.

In developing a set of coupons for monitoring the fatigue life of amember of any particular material, a thin sheet of the particularmaterial (preferably 0.5 mm thick) is obtained and a plurality ofelongated coupons approximately 25 mm by 6.5 mm are fabricated from thethin sheet of the material. Sets of five of the coupons are cut from thesheet and notches are cut into the five coupons in each set in themanner as is described above with respect to coupons 12, 14, 16, 18 and20.

After the sets of five notched coupons have been fabricated, fatiguetests are run on each coupon within each set to produce an S-n endurancecurve for each of the five different coupons to indicate the number ofcycles to develop fatigue failure in the coupons as a result of theapplied stress level. When the S-n curves have been produced for eachcoupon, they are examined to determine whether the separation betweenthe failure of the different coupons is adequate to provide the desiredseparation of the early fatigue failure warnings. If the separation issatisfactory, a new set of five coupons having the same notch pattern isproduced and each of the coupons are subjected to common fatigue testingunder controlled strain at various strain amplitudes to again verify theorder in which the coupons suffer fatigue failure. Thereafter, anotherset of the coupons may be fabricated and fatigue tested on a small-scalespecimen of the structural member to be monitored to verify theparticular portion or percentage of the fatigue life of the structuralmember at which each of the coupons will experience fatigue failure.

If, upon the initial testing it is found that the separation between theS-n curves of the coupons is not satisfactory and further refinementsand modifications of the coupon life are desired, the notch pattern onone or more of the coupons may be changed. Depending upon whether it isdesired to increase or decrease the fatigue life of a particular coupon,the wedge angle α could be modified or the orientation angle θ could bechanged, or both. As previously indicated, smaller wedge anglesrepresent more severe stress raisers and therefore shorter fatigue life.In addition, changes in the orientation angles θ represent differentcombinations of tension and shear on the line between the notch tips,resulting in varying sensitivity to the wedge angle and varying rates ofdecay of the stress near the notch tips. These parameters thus controlthe probability of growth of microdefects. Once the modifications havebeen made, further fatigue testing is conducted to generate new S-ncurves and the remainder of the above-described process is conducted.

From the foregoing description of a preferred embodiment it can be seenthat the present invention comprises a device for monitoring the fatiguelife of a structural member which is subject to fatigue failure. Thedevice contains no moving parts and is relatively simple and inexpensiveto produce but yet provides a good indication of the remaining fatiguelife of the structural member being monitored. The present inventionalso provides a method for developing a set of coupons for monitoringthe fatigue life of a structural member which may be made of any type ofmaterial. It will be recognized by those skilled in the art that changesmay be made to the above-described embodiment of the invention withoutdeparting from the broad inventive concepts thereof. It is understood,therefore, that this invention is not limited to the particularembodiment disclosed, but is intended to cover all modifications whichare within the scope and spirit of the invention as defined by theappended claims.

I claim:
 1. A method of making a device for monitoring the fatigue lifeof a member fabricated of a particular material comprising the stepsof:obtaining a thin sheet of the material from which the member isfabricated; cutting a plurality of generally parallel elongated couponsfrom the thin sheet of material; cutting a predetermined stressconcentrating notch pattern comprised of at least one pair of notchesinto each coupon, one notch of the notch pair being disposed on each ofthe longitudinal sides of a coupon, the notches of the notch pair beingsubstantially geometrically the same and substantially aligned with eachother, the notch pattern of each of the coupons varying in intensityfrom mild in which the edge surfaces of the notches are generally smoothand continuously curved, to severe, in which the edge surface of each ofthe notches exhibits a sudden change to form two portions which are indifferent planes, the two portions meeting to form a line so that uponthe application of the same strain to all of the coupons, each couponhas a different fatigue life, the fatigue life of each coupon being apercentage of the fatigue life of the member being monitored.
 2. Themethod as recited in claim 1 further including the steps of:testing eachof the coupons to develop and S-n endurance curve for each coupon whichindicates the number of cycles required to develop fatigue failurewithin the coupon; and comparing the S-n curves for the differentcoupons to determine whether the separation between the fatigue failureof the coupons is adequate to provide a desired separation of thefatigue failure warnings.
 3. The method as recited in claim 1 wherein atleast one of the coupons has a notch pattern comprised of at least onepair of wedge angle V-shaped notches, the wedge angle of the V-shapednotches being varied.
 4. The method as recited in claim 1 wherein theorientation angle of the notches is varied.
 5. The method as recited inclaim 1 wherein the number of notch pairs for each coupon is varied.